diff --git a/CHANGELOG.md b/CHANGELOG.md
index 129621e..d1b5216 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,14 @@ and this project adheres to
 ## [Unreleased]
 ## Changed
 - `Test.Hspec.GraphQL.*`: replace `IO` in the resolver with any `MonadCatch`.
+- The `Location` argument of `AST.Document.Definition.ExecutableDefinition` was
+  moved to `OperationDefinition` and `FragmentDefinition` since these are the
+  actual elements that have a location in the document.
+- `Validate.Rules` get the whole validation context (AST and schema).
+
+## Added
+- `Validate.Validation` contains data structures and functions used by the
+  validator and concretet rules.
 
 ## [0.9.0.0] - 2020-07-24
 ## Fixed
diff --git a/graphql.cabal b/graphql.cabal
index 73e71e7..46c3d6c 100644
--- a/graphql.cabal
+++ b/graphql.cabal
@@ -4,7 +4,7 @@ cabal-version: 1.12
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: 1d8c32c00a882ccd1fefc4c083d5fe4e83a1825fbf8e0dcfd551ff2c8cd2dda0
+-- hash: 59e2949d07cb5e678b493b77771db1bd64947de480f3da93ca07b3f2458cc495
 
 name:           graphql
 version:        0.9.0.0
@@ -50,6 +50,7 @@ library
       Language.GraphQL.Type.Out
       Language.GraphQL.Type.Schema
       Language.GraphQL.Validate
+      Language.GraphQL.Validate.Validation
       Test.Hspec.GraphQL
   other-modules:
       Language.GraphQL.Execute.Execution
diff --git a/src/Language/GraphQL/AST/Document.hs b/src/Language/GraphQL/AST/Document.hs
index 3394bfa..f60ddda 100644
--- a/src/Language/GraphQL/AST/Document.hs
+++ b/src/Language/GraphQL/AST/Document.hs
@@ -69,7 +69,7 @@ type Document = NonEmpty Definition
 
 -- | All kinds of definitions that can occur in a GraphQL document.
 data Definition
-    = ExecutableDefinition ExecutableDefinition Location
+    = ExecutableDefinition ExecutableDefinition
     | TypeSystemDefinition TypeSystemDefinition Location
     | TypeSystemExtension TypeSystemExtension Location
     deriving (Eq, Show)
@@ -84,13 +84,14 @@ data ExecutableDefinition
 
 -- | Operation definition.
 data OperationDefinition
-    = SelectionSet SelectionSet
+    = SelectionSet SelectionSet Location
     | OperationDefinition
         OperationType
         (Maybe Name)
         [VariableDefinition]
         [Directive]
         SelectionSet
+        Location
     deriving (Eq, Show)
 
 -- | GraphQL has 3 operation types:
@@ -195,7 +196,7 @@ type Alias = Name
 
 -- | Fragment definition.
 data FragmentDefinition
-    = FragmentDefinition Name TypeCondition [Directive] SelectionSet
+    = FragmentDefinition Name TypeCondition [Directive] SelectionSet Location
     deriving (Eq, Show)
 
 -- | Type condition.
diff --git a/src/Language/GraphQL/AST/Encoder.hs b/src/Language/GraphQL/AST/Encoder.hs
index a0dac5b..ba89d36 100644
--- a/src/Language/GraphQL/AST/Encoder.hs
+++ b/src/Language/GraphQL/AST/Encoder.hs
@@ -50,8 +50,8 @@ document formatter defs
     | Minified <-formatter = Lazy.Text.snoc (mconcat encodeDocument) '\n'
   where
     encodeDocument = foldr executableDefinition [] defs
-    executableDefinition (ExecutableDefinition x _) acc =
-        definition formatter x : acc
+    executableDefinition (ExecutableDefinition executableDefinition') acc =
+        definition formatter executableDefinition' : acc
     executableDefinition _ acc = acc
 
 -- | Converts a t'ExecutableDefinition' into a string.
@@ -68,12 +68,12 @@ definition formatter x
 -- | Converts a 'OperationDefinition into a string.
 operationDefinition :: Formatter -> OperationDefinition -> Lazy.Text
 operationDefinition formatter = \case
-    SelectionSet sels -> selectionSet formatter sels
-    OperationDefinition Query name vars dirs sels ->
+    SelectionSet sels _ -> selectionSet formatter sels
+    OperationDefinition Query name vars dirs sels _ ->
         "query " <> node formatter name vars dirs sels
-    OperationDefinition Mutation name vars dirs sels ->
+    OperationDefinition Mutation name vars dirs sels _ ->
         "mutation " <> node formatter name vars dirs sels
-    OperationDefinition Subscription name vars dirs sels ->
+    OperationDefinition Subscription name vars dirs sels _ ->
         "subscription " <> node formatter name vars dirs sels
 
 -- | Converts a Query or Mutation into a string.
@@ -190,7 +190,7 @@ inlineFragment formatter tc dirs sels = "... on "
     <> selectionSet formatter sels
 
 fragmentDefinition :: Formatter -> FragmentDefinition -> Lazy.Text
-fragmentDefinition formatter (FragmentDefinition name tc dirs sels)
+fragmentDefinition formatter (FragmentDefinition name tc dirs sels _)
     = "fragment " <> Lazy.Text.fromStrict name
     <> " on " <> Lazy.Text.fromStrict tc
     <> optempty (directives formatter) dirs
diff --git a/src/Language/GraphQL/AST/Parser.hs b/src/Language/GraphQL/AST/Parser.hs
index 687d8f5..7bc51cb 100644
--- a/src/Language/GraphQL/AST/Parser.hs
+++ b/src/Language/GraphQL/AST/Parser.hs
@@ -21,7 +21,8 @@ import Language.GraphQL.AST.DirectiveLocation
 import Language.GraphQL.AST.Document
 import Language.GraphQL.AST.Lexer
 import Text.Megaparsec
-    ( SourcePos(..)
+    ( MonadParsec(..)
+    , SourcePos(..)
     , getSourcePos
     , lookAhead
     , option
@@ -37,15 +38,11 @@ document = unicodeBOM
     *> lexeme (NonEmpty.some definition)
 
 definition :: Parser Definition
-definition = executableDefinition'
+definition = ExecutableDefinition <$> executableDefinition
     <|> typeSystemDefinition'
     <|> typeSystemExtension'
     <?> "Definition"
   where
-    executableDefinition' = do
-        location <- getLocation
-        definition' <- executableDefinition
-        pure $ ExecutableDefinition definition' location
     typeSystemDefinition' = do
         location <- getLocation
         definition' <- typeSystemDefinition
@@ -349,16 +346,22 @@ operationTypeDefinition = OperationTypeDefinition
     <?> "OperationTypeDefinition"
 
 operationDefinition :: Parser OperationDefinition
-operationDefinition = SelectionSet <$> selectionSet
+operationDefinition = shorthand
     <|> operationDefinition'
     <?> "OperationDefinition"
   where
-    operationDefinition'
-        = OperationDefinition <$> operationType
-        <*> optional name
-        <*> variableDefinitions
-        <*> directives
-        <*> selectionSet
+    shorthand = do
+        location <- getLocation
+        selectionSet' <- selectionSet
+        pure $ SelectionSet selectionSet' location
+    operationDefinition' = do
+        location <- getLocation
+        operationType' <- operationType
+        operationName <- optional name
+        variableDefinitions' <- variableDefinitions
+        directives' <- directives
+        selectionSet' <- selectionSet
+        pure $ OperationDefinition operationType' operationName variableDefinitions' directives' selectionSet' location
 
 operationType :: Parser OperationType
 operationType = Query <$ symbol "query"
@@ -412,13 +415,15 @@ inlineFragment = InlineFragment
     <?> "InlineFragment"
 
 fragmentDefinition :: Parser FragmentDefinition
-fragmentDefinition = FragmentDefinition
-    <$  symbol "fragment"
-    <*> name
-    <*> typeCondition
-    <*> directives
-    <*> selectionSet
-    <?> "FragmentDefinition"
+fragmentDefinition =  label "FragmentDefinition" $ do
+    location <- getLocation
+    _ <- symbol "fragment"
+    fragmentName' <- name
+    typeCondition' <- typeCondition
+    directives' <- directives
+    selectionSet' <- selectionSet
+    pure $ FragmentDefinition
+        fragmentName' typeCondition' directives' selectionSet' location
 
 fragmentName :: Parser Name
 fragmentName = but (symbol "on") *> name <?> "FragmentName"
diff --git a/src/Language/GraphQL/Execute/Execution.hs b/src/Language/GraphQL/Execute/Execution.hs
index d8d5b13..71a2baa 100644
--- a/src/Language/GraphQL/Execute/Execution.hs
+++ b/src/Language/GraphQL/Execute/Execution.hs
@@ -83,30 +83,6 @@ resolveAbstractType abstractType values'
             _ -> pure Nothing
     | otherwise = pure Nothing
 
-doesFragmentTypeApply :: forall m
-    . CompositeType m
-    -> Out.ObjectType m
-    -> Bool
-doesFragmentTypeApply (CompositeObjectType fragmentType) objectType =
-    fragmentType == objectType
-doesFragmentTypeApply (CompositeInterfaceType fragmentType) objectType =
-    instanceOf objectType $ AbstractInterfaceType fragmentType
-doesFragmentTypeApply (CompositeUnionType fragmentType) objectType =
-    instanceOf objectType $ AbstractUnionType fragmentType
-
-instanceOf :: forall m. Out.ObjectType m -> AbstractType m -> Bool
-instanceOf objectType (AbstractInterfaceType interfaceType) =
-    let Out.ObjectType _ _ interfaces _ = objectType
-     in foldr go False interfaces
-  where
-    go objectInterfaceType@(Out.InterfaceType _ _ interfaces _) acc =
-        acc || foldr go (interfaceType == objectInterfaceType) interfaces
-instanceOf objectType (AbstractUnionType unionType) =
-    let Out.UnionType _ _ members = unionType
-     in foldr go False members
-  where
-    go unionMemberType acc = acc || objectType == unionMemberType
-
 executeField :: (MonadCatch m, Serialize a)
     => Out.Resolver m
     -> Type.Value
diff --git a/src/Language/GraphQL/Execute/Transform.hs b/src/Language/GraphQL/Execute/Transform.hs
index 76d1fe7..9c7ad0a 100644
--- a/src/Language/GraphQL/Execute/Transform.hs
+++ b/src/Language/GraphQL/Execute/Transform.hs
@@ -255,18 +255,18 @@ defragment ast =
      in (, fragmentTable) <$> maybe emptyDocument Right nonEmptyOperations
   where
     defragment' definition (operations, fragments')
-        | (Full.ExecutableDefinition executable _) <- definition
+        | (Full.ExecutableDefinition executable) <- definition
         , (Full.DefinitionOperation operation') <- executable =
             (transform operation' : operations, fragments')
-        | (Full.ExecutableDefinition executable _) <- definition
+        | (Full.ExecutableDefinition executable) <- definition
         , (Full.DefinitionFragment fragment) <- executable
-        , (Full.FragmentDefinition name _ _ _) <- fragment =
+        , (Full.FragmentDefinition name _ _ _ _) <- fragment =
             (operations, HashMap.insert name fragment fragments')
     defragment' _ acc = acc
     transform = \case
-        Full.OperationDefinition type' name variables directives' selections ->
+        Full.OperationDefinition type' name variables directives' selections _ ->
             OperationDefinition type' name variables directives' selections
-        Full.SelectionSet selectionSet ->
+        Full.SelectionSet selectionSet _ ->
             OperationDefinition Full.Query Nothing mempty mempty selectionSet
 
 -- * Operation
@@ -324,8 +324,8 @@ selection (Full.InlineFragment type' directives' selections) = do
             case type' of
                 Nothing -> pure $ Left fragmentSelectionSet
                 Just typeName -> do
-                    typeCondition' <- lookupTypeCondition typeName
-                    case typeCondition' of
+                    types' <- gets types
+                    case lookupTypeCondition typeName types' of
                         Just typeCondition -> pure $
                             selectionFragment typeCondition fragmentSelectionSet
                         Nothing -> pure $ Left mempty
@@ -364,29 +364,17 @@ collectFragments = do
         _ <- fragmentDefinition nextValue
         collectFragments
 
-lookupTypeCondition :: Full.Name -> State (Replacement m) (Maybe (CompositeType m))
-lookupTypeCondition type' = do
-    types' <- gets types
-    case HashMap.lookup type' types' of
-        Just (ObjectType objectType) ->
-            lift $ pure $ Just $ CompositeObjectType objectType
-        Just (UnionType unionType) ->
-            lift $ pure $ Just $ CompositeUnionType unionType
-        Just (InterfaceType interfaceType) ->
-            lift $ pure $ Just $ CompositeInterfaceType interfaceType
-        _ -> lift $ pure Nothing
-
 fragmentDefinition
     :: Full.FragmentDefinition
     ->  State (Replacement m) (Maybe (Fragment m))
-fragmentDefinition (Full.FragmentDefinition name type' _ selections) = do
+fragmentDefinition (Full.FragmentDefinition name type' _ selections _) = do
     modify deleteFragmentDefinition
     fragmentSelection <- appendSelection selections
-    compositeType <- lookupTypeCondition type'
+    types' <- gets types
 
-    case compositeType of
-        Just compositeType' -> do
-            let newValue = Fragment compositeType' fragmentSelection
+    case lookupTypeCondition type' types' of
+        Just compositeType -> do
+            let newValue = Fragment compositeType fragmentSelection
             modify $ insertFragment newValue
             lift $ pure $ Just newValue
         _ -> lift $ pure Nothing
diff --git a/src/Language/GraphQL/Type/Internal.hs b/src/Language/GraphQL/Type/Internal.hs
index 9121d13..6f25777 100644
--- a/src/Language/GraphQL/Type/Internal.hs
+++ b/src/Language/GraphQL/Type/Internal.hs
@@ -8,6 +8,9 @@ module Language.GraphQL.Type.Internal
     ( AbstractType(..)
     , CompositeType(..)
     , collectReferencedTypes
+    , doesFragmentTypeApply
+    , instanceOf
+    , lookupTypeCondition
     ) where
 
 import Data.HashMap.Strict (HashMap)
@@ -89,3 +92,39 @@ collectReferencedTypes schema =
     polymorphicTraverser interfaces fields
         = flip (foldr visitFields) fields
         . flip (foldr traverseInterfaceType) interfaces
+
+doesFragmentTypeApply :: forall m
+    . CompositeType m
+    -> Out.ObjectType m
+    -> Bool
+doesFragmentTypeApply (CompositeObjectType fragmentType) objectType =
+    fragmentType == objectType
+doesFragmentTypeApply (CompositeInterfaceType fragmentType) objectType =
+    instanceOf objectType $ AbstractInterfaceType fragmentType
+doesFragmentTypeApply (CompositeUnionType fragmentType) objectType =
+    instanceOf objectType $ AbstractUnionType fragmentType
+
+instanceOf :: forall m. Out.ObjectType m -> AbstractType m -> Bool
+instanceOf objectType (AbstractInterfaceType interfaceType) =
+    let Out.ObjectType _ _ interfaces _ = objectType
+     in foldr go False interfaces
+  where
+    go objectInterfaceType@(Out.InterfaceType _ _ interfaces _) acc =
+        acc || foldr go (interfaceType == objectInterfaceType) interfaces
+instanceOf objectType (AbstractUnionType unionType) =
+    let Out.UnionType _ _ members = unionType
+     in foldr go False members
+  where
+    go unionMemberType acc = acc || objectType == unionMemberType
+
+lookupTypeCondition :: forall m
+    . Name
+    -> HashMap Name (Type m)
+    -> Maybe (CompositeType m)
+lookupTypeCondition type' types' =
+    case HashMap.lookup type' types' of
+        Just (ObjectType objectType) -> Just $ CompositeObjectType objectType
+        Just (UnionType unionType) -> Just $ CompositeUnionType unionType
+        Just (InterfaceType interfaceType) ->
+            Just $ CompositeInterfaceType interfaceType
+        _ -> Nothing
diff --git a/src/Language/GraphQL/Validate.hs b/src/Language/GraphQL/Validate.hs
index 5768615..95f7462 100644
--- a/src/Language/GraphQL/Validate.hs
+++ b/src/Language/GraphQL/Validate.hs
@@ -13,22 +13,19 @@ module Language.GraphQL.Validate
     , module Language.GraphQL.Validate.Rules
     ) where
 
-import Control.Monad.Trans.Reader (Reader, asks, runReader)
+import Control.Monad (foldM)
+import Control.Monad.Trans.Reader (Reader, asks, mapReaderT, runReader)
 import Data.Foldable (foldrM)
 import Data.Sequence (Seq(..), (><), (|>))
 import qualified Data.Sequence as Seq
 import Data.Text (Text)
 import Language.GraphQL.AST.Document
-import Language.GraphQL.Type.Schema
+import Language.GraphQL.Type.Internal
+import Language.GraphQL.Type.Schema (Schema(..))
 import Language.GraphQL.Validate.Rules
+import Language.GraphQL.Validate.Validation
 
-data Context m = Context
-    { ast :: Document
-    , schema :: Schema m
-    , rules :: [Rule]
-    }
-
-type ValidateT m = Reader (Context m) (Seq Error)
+type ValidateT m = Reader (Validation m) (Seq Error)
 
 -- | If an error can be associated to a particular field in the GraphQL result,
 -- it must contain an entry with the key path that details the path of the
@@ -48,36 +45,46 @@ data Error = Error
 
 -- | Validates a document and returns a list of found errors. If the returned
 -- list is empty, the document is valid.
-document :: forall m. Schema m -> [Rule] -> Document -> Seq Error
+document :: forall m. Schema m -> [Rule m] -> Document -> Seq Error
 document schema' rules' document' =
     runReader (foldrM go Seq.empty document') context
   where
-    context = Context
+    context = Validation
         { ast = document'
         , schema = schema'
+        , types = collectReferencedTypes schema'
         , rules = rules'
         }
     go definition' accumulator = (accumulator ><) <$> definition definition'
 
 definition :: forall m. Definition -> ValidateT m
 definition = \case
-    definition'@(ExecutableDefinition executableDefinition' _) -> do
+    definition'@(ExecutableDefinition executableDefinition') -> do
         applied <- applyRules definition'
         children <- executableDefinition executableDefinition'
         pure $ children >< applied
     definition' -> applyRules definition'
   where
-    applyRules definition' = foldr (ruleFilter definition') Seq.empty
-        <$> asks rules
-    ruleFilter definition' (DefinitionRule rule) accumulator
-        | Just message' <- rule definition' =
-            accumulator |> Error
-                { message = message'
-                , locations = [definitionLocation definition']
-                , path = []
-                }
-        | otherwise = accumulator
-    definitionLocation (ExecutableDefinition _ location) = location
+    applyRules definition' =
+        asks rules >>= foldM (ruleFilter definition') Seq.empty
+    ruleFilter definition' accumulator (DefinitionRule rule) =
+        flip mapReaderT (rule definition') $ \case
+            Just message' ->
+                pure $ accumulator |> Error
+                    { message = message'
+                    , locations = [definitionLocation definition']
+                    , path = []
+                    }
+            Nothing -> pure accumulator
+    ruleFilter _ accumulator _ = pure accumulator
+    definitionLocation (ExecutableDefinition executableDefinition')
+        | DefinitionOperation definitionOperation <- executableDefinition'
+        , SelectionSet _ location <- definitionOperation = location
+        | DefinitionOperation definitionOperation <- executableDefinition'
+        , OperationDefinition _ _ _ _ _ location <- definitionOperation =
+            location
+        | DefinitionFragment fragmentDefinition' <- executableDefinition'
+        , FragmentDefinition _ _ _ _ location <- fragmentDefinition' = location
     definitionLocation (TypeSystemDefinition _ location) = location
     definitionLocation (TypeSystemExtension _ location) = location
 
@@ -88,10 +95,21 @@ executableDefinition (DefinitionFragment definition') =
     fragmentDefinition definition'
 
 operationDefinition :: forall m. OperationDefinition -> ValidateT m
-operationDefinition (SelectionSet _operation) =
-    pure Seq.empty
-operationDefinition (OperationDefinition _type _name _variables _directives _selection) =
-    pure Seq.empty
+operationDefinition operation =
+    asks rules >>= foldM (ruleFilter operation) Seq.empty
+  where
+    ruleFilter definition' accumulator (OperationDefinitionRule rule) =
+        flip mapReaderT (rule definition') $ \case
+            Just message' ->
+                pure $ accumulator |> Error
+                    { message = message'
+                    , locations = [definitionLocation operation]
+                    , path = []
+                    }
+            Nothing -> pure accumulator
+    ruleFilter _ accumulator _ = pure accumulator
+    definitionLocation (SelectionSet _ location) = location
+    definitionLocation (OperationDefinition _ _ _ _ _ location) = location
 
 fragmentDefinition :: forall m. FragmentDefinition -> ValidateT m
 fragmentDefinition _fragment = pure Seq.empty
diff --git a/src/Language/GraphQL/Validate/Rules.hs b/src/Language/GraphQL/Validate/Rules.hs
index 9faaedd..a3b1a59 100644
--- a/src/Language/GraphQL/Validate/Rules.hs
+++ b/src/Language/GraphQL/Validate/Rules.hs
@@ -2,30 +2,99 @@
    v. 2.0. If a copy of the MPL was not distributed with this file, You can
    obtain one at https://mozilla.org/MPL/2.0/. -}
 
+{-# LANGUAGE ExplicitForAll #-}
+{-# LANGUAGE OverloadedStrings #-}
+
 -- | This module contains default rules defined in the GraphQL specification.
 module Language.GraphQL.Validate.Rules
-    ( Rule(..)
-    , executableDefinitionsRule
+    ( executableDefinitionsRule
     , specifiedRules
     ) where
 
+import Control.Monad (foldM)
+import Control.Monad.Trans.Class (MonadTrans(..))
+import Control.Monad.Trans.Reader (asks)
+import Control.Monad.Trans.State (evalStateT, gets, modify)
+import qualified Data.HashSet as HashSet
+import qualified Data.Text as Text
 import Language.GraphQL.AST.Document
-
--- | 'Rule' assigns a function to each AST node that can be validated. If the
--- validation fails, the function should return an error message, or 'Nothing'
--- otherwise.
-newtype Rule
-    = DefinitionRule (Definition -> Maybe String)
+import Language.GraphQL.Type.Internal
+import qualified Language.GraphQL.Type.Schema as Schema
+import Language.GraphQL.Validate.Validation
 
 -- | Default rules given in the specification.
-specifiedRules :: [Rule]
+specifiedRules :: forall m. [Rule m]
 specifiedRules =
     [ executableDefinitionsRule
+    , singleFieldSubscriptionsRule
     ]
 
 -- | Definition must be OperationDefinition or FragmentDefinition.
-executableDefinitionsRule :: Rule
+executableDefinitionsRule :: forall m. Rule m
 executableDefinitionsRule = DefinitionRule go
   where
-    go (ExecutableDefinition _definition _) = Nothing
-    go _ = Just "Definition must be OperationDefinition or FragmentDefinition."
+    go :: Definition -> RuleT m
+    go (ExecutableDefinition _) = lift Nothing
+    go _ = pure
+        "Definition must be OperationDefinition or FragmentDefinition."
+
+-- | Subscription operations must have exactly one root field.
+singleFieldSubscriptionsRule :: forall m. Rule m
+singleFieldSubscriptionsRule = OperationDefinitionRule go
+  where
+    go (OperationDefinition Subscription name' _ _ rootFields _) = do
+        groupedFieldSet <- evalStateT (collectFields rootFields) HashSet.empty
+        case HashSet.size groupedFieldSet of
+            1 -> lift Nothing
+            _
+                | Just name <- name' -> pure $ unwords
+                    [ "Subscription"
+                    , Text.unpack name
+                    , "must select only one top level field."
+                    ]
+                | otherwise -> pure
+                    "Anonymous Subscription must select only one top level field."
+    go _ = lift Nothing
+    collectFields selectionSet = foldM forEach HashSet.empty selectionSet
+    forEach accumulator (Field alias name _ directives _)
+        | any skip directives = pure accumulator
+        | Just aliasedName <- alias = pure
+            $ HashSet.insert aliasedName accumulator
+        | otherwise = pure $ HashSet.insert name accumulator
+    forEach accumulator (FragmentSpread fragmentName directives)
+        | any skip directives = pure accumulator
+        | otherwise = do
+            inVisitetFragments <- gets $ HashSet.member fragmentName
+            if inVisitetFragments
+               then pure accumulator
+               else collectFromSpread fragmentName accumulator
+    forEach accumulator (InlineFragment typeCondition' directives selectionSet)
+        | any skip directives = pure accumulator
+        | Just typeCondition <- typeCondition' =
+            collectFromFragment typeCondition selectionSet accumulator
+        | otherwise = HashSet.union accumulator
+            <$> collectFields selectionSet
+    skip (Directive "skip" [Argument "if" (Boolean True)]) = True
+    skip (Directive "include" [Argument "if" (Boolean False)]) = True
+    skip _ = False
+    findFragmentDefinition (ExecutableDefinition executableDefinition) Nothing
+        | DefinitionFragment fragmentDefinition <- executableDefinition =
+            Just fragmentDefinition
+    findFragmentDefinition _ accumulator = accumulator
+    collectFromFragment typeCondition selectionSet accumulator = do
+        types' <- lift $ asks types
+        schema' <- lift $ asks schema
+        case lookupTypeCondition typeCondition types' of
+            Nothing -> pure accumulator
+            Just compositeType
+                | Just objectType <- Schema.subscription schema'
+                , True <- doesFragmentTypeApply compositeType objectType ->
+                    HashSet.union accumulator<$> collectFields selectionSet
+                | otherwise -> pure accumulator
+    collectFromSpread fragmentName accumulator = do
+        modify $ HashSet.insert fragmentName
+        ast' <- lift $ asks ast
+        case foldr findFragmentDefinition Nothing ast' of
+            Nothing -> pure accumulator
+            Just (FragmentDefinition _ typeCondition _ selectionSet _) ->
+                collectFromFragment typeCondition selectionSet accumulator
diff --git a/src/Language/GraphQL/Validate/Validation.hs b/src/Language/GraphQL/Validate/Validation.hs
new file mode 100644
index 0000000..f6edc7a
--- /dev/null
+++ b/src/Language/GraphQL/Validate/Validation.hs
@@ -0,0 +1,34 @@
+{- This Source Code Form is subject to the terms of the Mozilla Public License,
+   v. 2.0. If a copy of the MPL was not distributed with this file, You can
+   obtain one at https://mozilla.org/MPL/2.0/. -}
+
+-- | Definitions used by the validation rules and the validator itself.
+module Language.GraphQL.Validate.Validation
+    ( Validation(..)
+    , Rule(..)
+    , RuleT
+    ) where
+
+import Control.Monad.Trans.Reader (ReaderT(..))
+import Data.HashMap.Strict (HashMap)
+import Language.GraphQL.AST.Document
+import Language.GraphQL.Type.Schema (Schema)
+import qualified Language.GraphQL.Type.Schema as Schema
+
+-- | Validation rule context.
+data Validation m = Validation
+    { ast :: Document
+    , schema :: Schema m
+    , types :: HashMap Name (Schema.Type m)
+    , rules :: [Rule m]
+    }
+
+-- | 'Rule' assigns a function to each AST node that can be validated. If the
+-- validation fails, the function should return an error message, or 'Nothing'
+-- otherwise.
+data Rule m
+    = DefinitionRule (Definition -> RuleT m)
+    | OperationDefinitionRule (OperationDefinition -> RuleT m)
+
+-- | Monad transformer used by the rules.
+type RuleT m = ReaderT (Validation m) Maybe String
diff --git a/tests/Language/GraphQL/AST/EncoderSpec.hs b/tests/Language/GraphQL/AST/EncoderSpec.hs
index 71ee948..5fa3706 100644
--- a/tests/Language/GraphQL/AST/EncoderSpec.hs
+++ b/tests/Language/GraphQL/AST/EncoderSpec.hs
@@ -123,7 +123,9 @@ spec = do
         it "indents block strings in arguments" $
             let arguments = [Argument "message" (String "line1\nline2")]
                 field = Field Nothing "field" arguments [] []
-                operation = DefinitionOperation $ SelectionSet $ pure field
+                operation = DefinitionOperation
+                    $ SelectionSet (pure field)
+                    $ Location 0 0
              in definition pretty operation `shouldBe` [r|{
   field(message: """
     line1
diff --git a/tests/Language/GraphQL/ValidateSpec.hs b/tests/Language/GraphQL/ValidateSpec.hs
index f84322d..c463dd9 100644
--- a/tests/Language/GraphQL/ValidateSpec.hs
+++ b/tests/Language/GraphQL/ValidateSpec.hs
@@ -148,7 +148,7 @@ validate queryString =
 
 spec :: Spec
 spec =
-    describe "document" $
+    describe "document" $ do
         it "rejects type definitions" $
             let queryString = [r|
               query getDogName {
@@ -169,3 +169,43 @@ spec =
                     , path = []
                     }
              in validate queryString `shouldBe` Seq.singleton expected
+
+        it "rejects multiple subscription root fields" $
+            let queryString = [r|
+              subscription sub {
+                newMessage {
+                  body
+                  sender
+                }
+                disallowedSecondRootField
+              }
+            |]
+                expected = Error
+                    { message =
+                        "Subscription sub must select only one top level field."
+                    , locations = [AST.Location 2 15]
+                    , path = []
+                    }
+             in validate queryString `shouldBe` Seq.singleton expected
+
+        it "rejects multiple subscription root fields coming from a fragment" $
+            let queryString = [r|
+              subscription sub {
+                ...multipleSubscriptions
+              }
+
+              fragment multipleSubscriptions on Subscription {
+                newMessage {
+                  body
+                  sender
+                }
+                disallowedSecondRootField
+              }
+            |]
+                expected = Error
+                    { message =
+                        "Subscription sub must select only one top level field."
+                    , locations = [AST.Location 2 15]
+                    , path = []
+                    }
+             in validate queryString `shouldBe` Seq.singleton expected